The present invention relates to ultrasonic working of workpieces for making structural parts, especially for producing ceramic tooth crowns, as, for example, dental veneers, inlays, crowns or bridges from dental ceramic material. Due to their favourable biological-chemical properties, their high tissue-tolerance and their low tendency for plaque-accumulation, ceramic tooth replacement materials are generally discussed as being material systems of top quality.
More specifically, the invention relates to a process of producing a ceramic tooth restoration by ultrasonic machining apparatus in which a receiver for a profiled sonotrode crown is positioned on each side of the workpiece to be worked, the receiver being in opposing relation to one another, and wherein in a first machining step, the workpiece is held by a workpiece holder, a first sonotrode crown is activated and one of the workpiece halves is machined to the desired shape by bringing the first said sonotrode crown into form-fitting engagement with said workpiece, and wherein in a second machining step the workpiece is held by the sonotrode crown by the form-fitting engagement, and a second sonotrode crown is activated for machining the remaining half of the workpiece to the desired shape. The invention also relates to an ultrasonic machining apparatus for carrying out the process.
The process as above described and the ultrasonic machining device for carrying out the process are known from DE 39 28 684 C2 (HAHN)(corresponding to U.S. application Ser. No. 07/678,367). Particular reference is additionally made to this patent publication and the prior art named therein.
Further, the publication "Keramikbearbeitung" (Working of Ceramics), Carl Hanser Verlag, Munchen 1989, pages 423-443 discloses a process and a device for working brittle materials, e.g. ceramic, glass, glass-ceramics etc., with the aid of ultrasound. The mechanical energy of the oscillating tool is transferred to a lapping mixture in a working gap between the sonotrode crown and the tool, which leads to a formation of chip and finally to a projection of the sonotrode crown in the workpiece. However, these methods are restricted to the forming of one workpiece surface. The necessary shaping tools are made by conventional metal-cutting methods, e.g. turning on a lathe, milling, drilling, etc., or by electrical discharge machining. Moreover, geometrically complex shapes of small dimensions frequently cannot be made from conventional workpiece materials.
DE 40 29 285 A1 (SIEMENS) describes a sonotrode having its shaping part, i.e. its sonotrode crown, made at least partially from silicon. This choice of material serves to optimise the wear behavior. It furthermore facilitates the use of the so-called microstructure technique (Mikrostrukturtechnik) for forming the working surface of the sonotrode crown. The microstructure technique is known from the field of manufacture of semiconductor elements and comprises, for example, photolithography and isotropic and anisotropic etching methods.
The above-mentioned electrical discharge machining process for making structural parts for the working of dental prostheses is known, for example, from DE 37 35 558 C2 (HERAEUS) and DE 35 44 123 C2 (WALTER). The disadvantages thereof have been discussed in the above-mentioned DE 39 28 648 C2 (HAHN).
Due to the unavoidable and partly significant wear and the short tool life of the sonotrode crowns resulting therefrom, conventional precise mechanical or microelectronic manufacturing processes or manufacturing processes by electrical discharge machining are not economical for the making of sonotrode crowns.
Finally, DE 36 06 305 A1 (HANSEN) describes an ultrasonic machining tool comprising an ultrasonic generator (ultrasonic converter including amplifier) and a sonotrode. The sonotrode is fixedly clamped to the ultrasonic generator by a differential screw and centered by cylindrical shoulders. The sonotrode can thereby easily be substituted without necessitating a new adjustment of the workpiece.